1. Field of the Invention
The invention relates to aircraft gas turbine engine electrical generators and, more particularly, to aircraft gas turbine engine electrical generators with counter-rotatable rotors.
2. Description of Related Art
An aircraft gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor, and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor or spool. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor. The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor or spool. The low pressure shaft extends through the high pressure rotor.
Some fan jet engines have been designed with counter-rotating turbines that power counter-rotating fans and boosters or low pressure compressors. U.S. Pat. Nos. 4,790,133, 4,860,537, 5,307,622 and 6,732,502 disclose counter-rotating low pressure turbines (LPT) that power counter-rotating fans and booster or low pressure compressors. Most of the thrust produced is generated by the fan. There are also various designs for counter-rotating fan engines that use gearboxes to effect counter-rotation of the fans and boosters. Counter-rotating fans, boosters, and turbines greatly enhance the engine's fuel efficiency. Conventional boosters are extremely inefficient at low speeds and result in higher stage counts to achieve reasonable efficiencies. Aircraft turbofan gas turbine engines with counter-rotating boosters have been developed to provide highly efficient boosters with a relatively low number of booster stages.
Aircraft systems are increasingly being designed to accommodate greater and greater electrical loads. Electrical power is extracted mechanically from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting electrical power from the engine mechanical power may reduce the ability to operate the engine properly. Current turbofan aircraft gas turbine engines requiring higher demands for aircraft power utilize very large generators mounted to a core (high pressure rotor) driven gearbox. At flight conditions where lower thrust and high electric power is required, such as approach, the core must run fast to keep up the electrical power demand but the result is more thrust which must be spoiled to maintain approach speeds, thus, reducing fuel efficiency.
Electrical power may be extracted from the high-pressure (HP) engine spool in a gas turbine engine. The relatively high operating speed of the HP engine spool makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, it is desirable to draw power from other sources within the engine, rather than to rely solely on the HP engine spool to drive the electrical generators. The low-pressure (LP) engine spool provides an alternate source of power transfer, however, the relatively lower speed of the LP engine spool typically requires the use of a gearbox, as slow-speed electrical generators are often larger than similarly rated electrical generators operating at higher speeds. Electrical generators operating off the LP rotor or spool require large generators due to low speeds of the LP rotor or gearboxes to increase speed. These generators are a simple rotor and stator configuration.
Therefore, there is a need for an electrical generator for aircraft turbofan gas turbine engines that can be used to provide a great amount of electrical power over a wide range of engine operating speeds.